large low-speed facility (llf) - dnw
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Large Low-speed Facility (LLF)
DNW - LLF
About us
The Foundation DNW (German-Dutch Wind Tunnels) was
established in 1976 by the Dutch National Aerospace Laboratory
(NLR) and the German Aerospace Center (DLR), as a non-profit
organization under Dutch law. DNW owns the largest low-speed
wind tunnel in Europe, the LLF, and operates the aeronautical
wind tunnels of DLR and NLR, which are fully integrated in the
DNW organization.
The main objective of DNW is to provide its customers from
the research community and aerospace industry with a wide
spectrum of wind tunnel test- and simulation techniques, operated
by one organization, providing the benefits of resource sharing,
technology transfer, and coordinated implementation of research
and development results.
Test section (m) 9.5x9.5 8x6 closed
8x6 open 6x6
Maximum velocity (m/s), empty test section
60 115 80 140
Maximum velocity (m/s), with model at sting support
55 105 80 130
Reynolds number (x 106 for reference length 1 m)
3.9 5.4 3.8 5.9
Key Technical ParametersMach number range 0.01 – 0.42
Pressure and temperature ambient
Turbulence level (at model center) lon: 0.02%, lat: 0.04%
Flow uniformity ∆ cp = 0.001 (5Pa)
Flow angularity < 0.06 ̊
Temperature uniformity < 0.2 ̊ C
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DNW - LLF
The LLF (Figure 1) is a closed return circuit wind
tunnel (with open and closed test sections)
for industrial aerodynamic and aero-acoustic
testing of complete aircraft configurations or its
components. DNW offers wind tunnel testing
techniques for the aerodynamic, aero-acoustic
and aero-elastic simulation of aircraft models in
a controlled environment with excellent air flow
characteristics. The experimental capabilities
of the LLF capture the essence of the issues to
be investigated in the low speed regime, i.e.
aircraft take-off and landing. Innovations and
investments focus on aircraft safety (flight in
ground proximity), environmental issues (noise
hindrance) and engine integration related testing
capabilities (saving fuel, reducing CO2 emission).
Typical issues addressed during wind tunnel
testing in the LLF cover aircraft configuration
studies, aerodynamic database creation (civil
and military transport aircraft, fighters, helicop-
ters, spacecraft, cars and trucks), engine inte-
gration studies with turbine-powered simulators
(turbofan and propeller) and engine air intake
and exhaust jet and aeroacoustic investigations.
Customer Benefits
Low Noise AircraftMost of the noise generated in the vicinity of
airports is produced by aircraft approaching or
taking-off, taxiing along runways and by engine
testing. DNW supports the strategy of the
aviation industry to develop quiet aircraft in a
continuous effort to limit aircraft noise impact,
by offering an excellent test environment to
reduce noise at source. DNW’s state-of-the-art
Figure 1LLF wind tunnel circuit
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DNW - LLF
technologies for measuring aircraft noise and
the related reduction mechanisms for full-size
components (Figure 2) or full-span models reveal
detailed knowledge of aircraft noise sources.
The large open test section of the LLF and
semi-anechoic test hall (50 m x 30 m x 20 m)
are specifi cally qualifi ed for aeroacoustic inves-
tigations due to their anechoic quality and very
low background noise level.
Safe Take-off and LandingTo realistically represent the infl uence of the
ground proximity (Figure 3), especially during
aircraft landing with wing fl aps and slats fully
deployed, the moving belt ground plane has been
upgraded. The original fl exible belt moving at
speeds of up to 40 m/s was replaced by a steel
belt system capable of running up to 80 m/s.
A sophisticated suction/blowing system keeps
the belt absolutely fl at, also under large aerody-
namic loading (when testing aircraft in ground
proximity). A boundary layer removal system and
reinjection scoop complement the test set-up.
Effi cient Engine Aircraft – engine integration is of crucial
importance for a successful aircraft design.
DNW has specialized in providing simulation
solutions to accurately assess the impact on
engine infl ow conditions, aircraft performance &
stability and thrust reverser effectiveness. DNW
owns several air–driven Turbofan Propulsion
Simulators (TPS) units for different model scales,
representing turbofan engines of all major engine
manufacturers (Figure 4). The compressed air
plant of the LLF delivers 4.7 kg/s (6.6 lb/s)
continuously at 80 bar for powering TPS units.
Application of a unique airline bridge design in
the wind tunnel model allows for accurate and
interference-free measurement of TPS thrust
levels. Calibration of these systems can be done
in-house in the Engine Calibration Facility ECF.
Figure 2Landing gear acoustics (EU SILENCE® project)
Figure 4Counter rotating open rotor model (EU CleanSky Z08 project)
Figure 3Business jet model in ground effect (EU CleanSky PLAAT project)
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DNW - LLF
Wind Tunnel
Test SectionsThe larger of the two closed test sections has a
cross section of 9.5 m x 9.5 m and the smaller
has a cross section of 8 m x 6 m (26.2 ft x
19.7 ft). This test section can be converted to
6 m x 6 m (19.7 ft x 19.7 ft). For the open jet
arrangement the 8x6 nozzle is used together
with the 9.5 m x 9.5 m transition. This open jet
test section is part of the so-called test hall
(50 m x 30 m 20 m), which is completely
covered with sound-absorbing lining material
during acoustic tests.
Model Support SystemsThe fl exibility of the LLF, especially its modular
construction with a range of different test
sections and model support systems, enables
the realization of a variety of different test
set-ups and wind tunnel speed ranges to suit
the customer’s needs.
A typical aircraft model wing span is 4.5 m and
the maximum tolerable model weight is 1500 kg.
To optimize testing time, a range of remote
controls for aircraft control surfaces is available
for integration into the wind tunnel model.
The LLF offers model mounting capabilities
on a sting support system (ventral or dorsal)
via an internal balance (typical for aircraft and
helicopter testing), an underfl oor six-component
external platform balance, e.g. used for full-size
cars/trucks (Figure 5) and semi-span models,
and different support systems for open jet
testing. The sting support allows for 360 ̊ model
roll angle variation for models of limited weight
and has a 6.6 m vertical stroke. The model angle
of attack setting range is 56 ̊ and the yawing
angle is 50 ̊ (angle accuracy 0.02 ̊).
The automated model control system allows for
data acquisition in continuous measuring mode
for aerodynamic model runs (data points at 0.4 ̊) and in step-by-step mode for acoustic testing.
Auxiliary Systems• Rotor test stand (owned and operated by DLR)
• Vacuum systems for engine air intake simula-
tion (5 kg/s mass fl ow rate at 95 bar, that can
be doubled with the use of nitrogen).
Figure 5MAN TGX truck testing
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DNW - LLF
Figure 6EMBRAER KC-390 optical measurements
Simulation Techniques
The wall correction routine of LLF for lift inter-
ference and for solid blockage uses the definitions
and formulation of the AGARDograph 109. For
the wake blockage the Maskell-Vayssaire ideal-
polar-method adopted for an on-line application
is used.
Measurement Techniques
It’s DNW’s policy to keep measurement equip-
ment at a high standard. Sharing of equipment
between the various DNW wind tunnel sites is
common practice. This allows meeting a large
range of customer requirements. In addition,
the support by both parent organizations
(German Aerospace Center DLR and Netherlands
Aerospace Center NLR) gives DNW access to
experience, knowledge, innovations and new
developments.
Forces and MomentsFor the measurement of static forces and
moments, a wide range of internal strain gauge
load balances of different size and load range is
available (length: 700 to 1250 mm, diameter:
150 to 220 mm). Of course also customer
supplied balances can be applied.
The external 7 m diameter platform balance has
a load range of 20 – 65 kN / 20 – 40 kNm.
The on-board strain gauge measurement system
is capable of measuring 80 auxiliary strain gauge
balance components/signals.
PressuresFor static pressure measurements Scanivalve©
Scanning Systems are operated. These allow the
simultaneous measurement of 1500 pressures
accurately and fast, by using temperature-
compensated electronic pressure scanning
modules (0,1% accuracy of the ZOC module
range; ranges 1, 5, 15, 50 psi = 6.9, 34.5,
103, 345 kPa). For highly accurate pressure
measurements, differential pressure transducers
are available. For the evaluation of dynamic
surface flow pressures up to 240 Endevco©
and/or Kulite© transducers can be served by
GBM Viper data acquisition systems. Force range 6.500 – 50.000 N
Moment range 3.000 – 15.000 Nm
Uncertainty (3 sigma) < 0.3%
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DNW - LLF
Flow FieldInstantaneous flow field images can be recorded
by means of the Particle Image Velocimetry
(PIV) technique in an arbitrary plane of the flow.
One (for 2D) or two (for 3C) digital cameras take
images of a plane in a laser light sheet (Figure
6). The light sheet is erected within the flow of a
wind tunnel in order to illuminate tracer particles.
These are seeded into the flow upstream of the
flow phenomenon to be investigated.
Figure 7Open test section background noise measurements
The illuminated tracer particles within the light
sheet window (30 x 40 cm) are recorded twice,
allowing the calculation of particle displacement
by cross correlation of a pair of images. A high
productivity is achieved by mounting laser equip-
ment and cameras on a traversing sledge in the
tunnel circuit or outside, since the test section
walls are equipped with large glass windows.
A stereoscopic point tracking technique is used
to measure changes in the position (e.g. rudder
deflection or rotor blade flapping) or shape of an
object (wing bending, refueling hose). It measu-
res marker positions on a (rotating) wind tunnel
model component to derive its deformation or
position. With two or more digital cameras, the
spatial position of markers is determined. For a
typical model scale of 5000 mm, the coordinate
detection accuracy is approximately 0.3 mm or
0.1 degree in wing twist on a full span model.
Several other techniques can be applied like:
• Pressure Sensitive Paint (PSP) to quantify wind
tunnel model surface pressure distribution
• Laser Light Sheet (LLS) to visualize flow field
features
• Infrared (IR) technique to observe laminar-
turbulent flow transition on a model surface
• Temperature Sensitive Paint (TSP)
AcousticsThe LLF was designed as a low noise facility.
Faculty upgrades in the between 2010 and 2015
on corner vanes, flow straightener and the
anechoic quality of the test hall reduced the
background noise of the wind tunnel significantly
to levels below 65 dB for frequencies above
800 Hz (Figure 7).
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DNW - LLF
For acoustic measurements three independent
microphone systems are available:
• 160 Far-field microphones at fixed positions
on the walls and ceiling of the test hall (mostly
of the electret type)
• 60 Microphones installed on traversing
mechanisms, wall and floor of in the open test
hall (½” free-field condenser microphone)
• Two out-of-flow microphone arrays of each
140 electret microphones installed in a fixed
frame (4 m x 4 m)
• Two wall-installed in-flow microphone arrays
with each 144 electret microphones installed
in a fixed frame (1 m x 1 m) for application in
a closed test section
Measurement techniques focus on the source
and the nature of the noise and are capable
of distinguishing between individual sources,
thanks to diagnostic acoustic arrays consisting
of multiple microphones. Correlation and phase
analysis of the signals of these arrays enable the
strength and location of relevant noise sources
to be determined.
A phased array beamforming technique is
applied to determine the locations and strength
of individual sound sources (typical resolution
is provided in the table below). CLEAN-SC
deconvolution algorithms including convection
and shear layer refraction corrections are
applied.
Acoustic array results are typically graphically
presented as contour plots, showing the
measured noise sources on the model for
different frequencies.
Data Acquisition
Dynamic data (including acoustics) are acquired
by five 48-channel (16 bit, 200 kHz sampling
rate per channel) high precision GBM Viper
systems, synchronized with the wind tunnel
or model control system and connected to the
static data acquisition system.
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